Check this out, it was what inspired Deb's design but if you don't sew........
http://www.rivbike.com/webalog/baggage_racks/20003.html


Another idea are the topeak wedge bags. They come in 2 types, and the style that has straps rather than clicking onto a bracket are shaped so they go more down than back. I have the large one and its back is shorter than the rear of my seat, a smaller one may be better, but they do give the dimensions so you could take some measurements.
http://www.biketiresdirect.com/produ...7502&c=7179159